Detection of Fast Neutrons
The efficient detection of fast neutrons, especially over a large area (e.g. >10 cm×10 cm) is currently technologically limited. The common methods for efficient detection of fast neutrons are via interactions with the hydrogen nucleus (i.e. a proton) in a solid or liquid medium. The hydrogen nucleus is important because it has a high interaction probability (i.e. large neutron cross-section) for fast neutrons. Furthermore, because of its low mass, the interaction of the neutron with the hydrogen nucleus produces a recoiling charged proton with high average energy because of reaction kinematics. Thus, the neutron loses much more of its energy in a collision with the hydrogen nucleus than in a collision with a heavier nucleus; this energy loss is transferred to the proton, making it relatively easy to detect using modern conventional radiation detectors.
Several solid hydrogenous scintillators are commonly used for efficient fast neutron detection. Examples include ordinary plastic scintillators, stilbene crystals, doped p-terphenol crystals and anthracene crystals. The reason that these are all organic type materials is that all scintillation detectors also respond to ubiquitous gamma rays in our everyday environment. Since gamma rays interact strongly with materials of high atomic number (Z), neutron detectors that are based on organic (low Z) material have the least response to gamma rays, which could otherwise mask the neutron signal. Fortunately, all of the above scintillators, except the plastic scintillator, also have the useful property that neutron and gamma-ray interactions lead to luminescence signals with different decay times. By using fast timing electronics, it is possible to separate the neutron and gamma-ray signals, i.e. discriminate gamma ray signals in favour of neutron signals. However, plastic scintillators do not possess this timing characteristic, making them of limited use for most neutron detection applications.
The most widely-used fast neutron detector with good detection efficiency is a liquid hydrogenous scintillator, known under the commercial trade-names of NE-213 or BC-501. These are chemical mixtures of xylene, naphthalene and wavelength-shifters. These detectors have good detection efficiency and good timing properties to allow for neutron/gamma discrimination.
All the above detectors are scintillators, which imply that they need a method to convert the scintillations (short flashes of light) into an electronic signal that can be processed and analyzed. The most common method of achieving this conversion is to use a photomultiplier tube (PMT). The PMT is a sealed tubular glass assembly under high vacuum. On the front end of the assembly is a photocathode—a very thin layer of a compound (often an alkali compound) that has a low work function and high secondary electron emission. Following the photocathode is a structure (dynodes) whose purpose is to amplify the electrons from the photocathode into a large enough electronic pulse for analogue and digital analysis. Thus, neutron interactions in the scintillator produce light, which is directed towards the PMT. This light penetrates the glass layer on the front of the PMT to impinge the photocathode substrate which leads to secondary electron emission. Under an applied high voltage, these secondary electrons are amplified by the dynode structure to produce an electronic signal at the anode, located on the back end of the PMT.
Aside from the use of scintillators, there is another approach where neutron scattering with hydrogen is used as the basis of a fast neutron detector. This utilizes a thin layer (radiator foil) of hydrogenous material (e.g. polyethylene) directly in front of a charged particle detector (e.g. a silicon diode). When neutrons interact with hydrogen in the radiator foil, many of the protons, scattered in the forward direction, escape from the foil to impinge the charged particle detector and are thus counted. The detection efficiency of this approach is limited by the maximum thickness of foil that permits the scattered protons to escape (for 2.5 MeV neutrons, thickness is ≦100 μm) and the area of the proton detector. Silicon diode detectors are often the proton detector of choice. These detectors are commonly only 1 to 5 cm2 because larger-area detectors suffer from excessive electronic noise due to the larger capacitance. These constraints limit the achievable radiator detection efficiency to less than ˜1%. The use of proton radiators has mainly been used for small detection systems.
There are also several fast neutron detectors that are based on various types of gases (e.g. hydrogen-filled counters, 3He counters, BF3 counters, 4He recoil spectrometers, gaseous scintillation detectors, etc.). However, because these detectors are in the gaseous phase, the molecular density of these types of detectors is much lower than solid or liquid phase detectors, resulting in low detection efficiency. This attribute of gaseous counters makes them irrelevant for the current discussion.
There is another class of neutron detectors that detect lower-energy (i.e. “thermal” neutrons), rather than fast neutrons. While this fact alone may appear to eliminate them from further consideration, there are various designs of fast neutron detectors that actually utilize thermal neutron detectors. This apparent contradiction is clarified by explaining that such designs make use of hydrogenous moderators (e.g. polyethylene) to convert fast neutrons to thermal neutron (via hydrogen scattering) so that the thermal neutron detectors can be used as the sensor. However, the use of a neutron moderator to convert fast neutrons to thermal neutrons is generally an inefficient process. Except for special counting geometries, this approach cannot yield detection efficiencies beyond a few percent. Furthermore, the thermalization process takes up to a millisecond to convert a fast neutron to a thermal neutron; so such detectors cannot be used for reasonably prompt counting of fast neutrons. Also, the thermal neutrons migrate significant distances in the moderator material, making such detectors not appropriate for good imaging applications.
More complete discussions of neutron detection technologies, photomultipliers and silicon diode detectors are given in standard reference books on radiation detection (e.g. G. F. Knoll, Radiation Detection and Measurements third edition (John Wiley & Sons, United States (2000)).
Limitations of Existing Fast Neutron Detection Technologies for Large Area Detection and Attempts at Improvements
There are many applications where it is desirable to have a large-area, efficient fast neutron detector. One main application is in connection with fast neutron radiography, of interest to homeland security or medical physics applications. Fast neutrons are particularly suited for imaging low Z materials, such as explosives, narcotics, or human tissue.
It is currently possible to meet this requirement by constructing a large matrix array of fast neutron scintillators viewed with individual photomultipliers (PMTs). However, a typical single detector element might measure 5 cm×5 cm×5 cm thick. A modest area of, say, 30 cm×30 cm would then require 36 detectors. A larger area of 1 m×1 m would require 400 detectors. Such an approach can be extremely expensive (each PMT alone costs several hundred dollars) and physically complex. A slightly different configuration could involve a large flat reservoir of liquid scintillator viewed by a matrix of independent PMTs through a glass wall on the backside of the reservoir. However, the cost and complexity of such a system would not be significantly reduced. It would also be possible to use a large slab of plastic scintillator (plastic scintillators are not expensive) viewed with a matrix of photodiodes (photodiodes are much more economical than PMTS). Such an assembly is less costly, but this system would not be able to discriminate against ubiquitous gamma rays by pulse-shape discrimination.